Abstract: Immunophenotyping Core A central unifying hypothesis of this research program is that MCC ? both viral and UV-damage associated - is a highly immunogenic tumor and, consequently, amenable to immune-oncologic intervention. This hypothesis is supported by the presence of virus-specific T cells and antibodies in many of the patients as well as an approximate 50% ORR in metastatic MCC patients treated with monotherapy anti-PD-1 or anti-PD-L1 monoclonal antibodies (mAbs). Unfortunately, many patients still fail to respond to PD-1 blockade. The overarching goal of the Immunophenotyping Core is to provide cutting-edge, slide-based technologies to interrogate the tumor microenvironment, in particular, to apply multiparametric immunohistochemistry (mIHC) to understanding the critical cell-cell interactions occurring within the tumor microenvironment (TME) that either enable or disable productive anti-tumor immune responses. We anticipate that these mechanistic insights will inform future combination immune-oncology trials in MCC patients.